ECV - Atmospheric Physics

Last modified on Sept 10, 2025 08:44

Contributors: C. Crevoisier (Laboratoire de Météorologie Dynamique (LMD)/CNRS), N. Meilhac (FX-CONSEIL/Laboratoire de Météorologie Dynamique (LMD))

Issued by: Laboratoire de Météorologie Dynamique/CNRS, France

Date: 19/11/2024

Ref: C3S2_313a_DLR_WP1-DDP-GHG-v1_PUGS_MTCO2_v10.1_MTCH4_v10.2

Official reference number service contract: 2024/C3S2_313a_DLR/SC1 

History of modifications

List of datasets covered by this document

Acronyms

General definitions

Essential climate variable (ECV): An ECV is a physical, chemical, or biological variable or a group of linked variables that critically contributes to the characterization of Earth's climate (Bojinski et al., 2014).

Climate data record (CDR): The US National Research Council (NRC) defines a CDR as a time series of measurements of sufficient length, consistency, and continuity to determine climate variability and change (National Research Council, 2004).

Fundamental climate data record (FCDR): A fundamental climate data record (FCDR) is a CDR of calibrated and quality-controlled data designed to allow the generation of homogeneous products that are accurate and stable enough for climate monitoring.

Thematic climate data record (TCDR): A thematic climate data record (TCDR) is a long time series of an essential climate variable (ECV) (Werscheck, 2015).

Intermediate climate data record (ICDR): An intermediate climate data record (ICDR) is a TCDR which undergoes regular and consistent updates (Werscheck, 2015), for example because it is being generated by a satellite sensor in operation.

Satellite data processing levels: The NASA Earth Observing System (EOS) distinguishes six processing levels of satellite data, ranging from Level 0 (L0) to Level 4 (L4) as follows (Parkinson et al., 2006).

Absolute systematic error or systematic error: Component of measurement error that in replicate measurements remains constant or varies in a predictable manner. Note that "systematic error" refers to the absolute systematic error (in contrast to "relative systematic error" defined below). For satellite GHG ECV products especially the relative systematic error is important.

Relative systematic error, relative accuracy or relative bias: Identical with "Systematic error" but after bias correction and without considering a possible global offset (overall mean bias). Reflects the importance of spatially and temporally correlated errors (spatio-temporal biases). Computed from standard deviations of spatial and temporal biases. 

Bias: Estimate of a systematic measurement error.

Precision: Measure of reproducibility or repeatability of the measurement without reference to an international standard so that precision is a measure of the random and not the systematic error. Suitable averaging of the random error can improve the precision of the measurement but does not establish the systematic error of the observation (CMUG-RBD, 2012).
Note: Precision is quantified with the standard deviation (1-sigma) of the error distribution.

Stability: Term often invoked with respect to long-term records when no absolute standard is available to quantitatively establish the systematic error - the bias defining the time-dependent (or instrument-dependent) difference between the observed quantity and the true value (CMUG-RBD, 2012).
Note: Stability requirements cover inter-annual error changes. If the change in the average bias from one year to another is larger than the defined values, the corresponding product does not meet the stability requirement.

Representativity: Extent to which an average of a set of measured values corresponds to the true average, e.g., over a grid cell. It is important when comparing with or assimilating in models. Measurements are typically averaged over different horizontal and vertical scales compared to model fields. If the measurements are smaller scale than the model it is important. The sampling strategy can also affect this term (CMUG-RBD, 2012).

Threshold requirement: The threshold is the limit at which the observation becomes ineffectual and is not of use for climate-related applications (CMUG-RBD, 2012).

Goal requirement: The goal is an ideal requirement above which further improvements are not necessary (CMUG-RBD, 2012).

Breakthrough requirement: The breakthrough is an intermediate level between the "threshold" and "goal" requirements, which - if achieved - would result in a significant improvement for the targeted application. The breakthrough level may be considered as an optimum, from a cost-benefit point of view when planning or designing observing systems (CMUG-RBD, 2012).

Horizontal resolution: Area over which one value of the variable is representative of (CMUG-RBD, 2012). 

Vertical resolution: Height over which one value of the variable is representative of. Only used for profile data (CMUG-RBD, 2012). 

Observing Cycle (or Revisit Time): Temporal frequency at which the measurements are required (CMUG-RBD, 2012).

Executive summary

This document is a Product User Guide and Specification (PUGS) generated in the framework of the Copernicus Climate Change Service (C3S, https://climate.copernicus.eu/). For C3S a large number of satellite-derived Essential Climate Variable (ECV) data products are generated and made available via the Copernicus Climate Data Store (CDS, https://cds.climate.copernicus.eu/). 

This document is the user guide for two satellite-derived atmospheric C3S data products, MTCO2_OBS4MIPS and MTCH4_OBS4MIPS, for the two main anthropogenic greenhouse gases (GHG) carbon dioxide (CO₂) and methane (CH₄), respectively. These products are mid-tropospheric-averaged air mixing ratios (mole fractions) of CO₂ and CH₄ products (noted as MT-CO₂ and MT-CH₄) derived from observations made by the simultaneous observations of IASI (Infrared Atmospheric Sounding Interferometer) and AMSU-A (Advanced Microwave Sounding Unit A) instruments onboard the European Metop-A (July 2007-August 2021), Metop-B (February 2013-December 2021) and Metop-C (since May 2019) platforms.

IASI is a high resolution Fourier Transform Spectrometer based on a Michelson Interferometer coupled to an integrated imaging system that measures infrared radiation emitted from the Earth (https://iasi.cnes.fr/en/IASI/index.htm). Also flying onboard Metop satellites is the AMSU-A instrument, which is a 15-channel microwave radiometer, which measures scene radiances in 15 discrete frequency channels spanning 23-90 GHz. AMSU-A is used to provide temperature profile information about the observed situation.

The combined use of both Metop-A and -B satellites, which followed the same orbit but with nearly half an orbit out of phase, yielded a complete coverage of the Earth in one day until the end of August 2021, when Metop-A was decommissioned. With the launch of Metop-C in 2018, and declared operational in May 2019, these time series will cover a period of about 20 years at the end of Metop-C lifetime allowing the study of trends and rates of change of CO₂ and CH₄ atmospheric mixing ratios in the mid-troposphere.

The MTCO2_OBS4MIPS and MTCH4_OBS4MIPS products are merged multi-sensor MT-CO₂ and MT-CH₄ Level 3 (L3) products with 1^ox1^o spatial resolution daily, generated from individual satellite sensor Level 2 (L2) MT-CO₂ and MT-CH₄ products from Metop-A, -B and -C available between July 2007 and December 2023 (updated every 6 months). Previous versions of this dataset included only L2 products.

Both the input data and the algorithm used to generate the product are described in detail in ATBD MTGHG, 2024.

1. Product Description

In this section an overview of the MTCO2_OBS4MIPS v10.1 and MTCH4_OBS4MIPS v10.2 data products- specified in terms of variable, associated characteristics, processing level(s) and instrument(s) - is given. 

Each of these products presented here are retrieved from simultaneous observations of the IASI and AMSU instruments flying together onboard the three successive Metop-A, -B and -C satellites by using a non-linear inference scheme (NLIS) using Multi-Layer Perceptrons (see ATBD MTGHG, 2024 for more details) . IASI hyperspectral observations in the thermal infrared at 7.7 µm (resp. 15 µm), which are sensitive to both temperature and gas concentrations of CH₄ (resp. CO₂), are used in conjunction with microwave observations from the AMSU-A instruments, only sensitive to temperature, to decorrelate both signals (Crevoisier et al., 2009a, 2009b, 2013). Retrievals are thus performed at the AMSU field-of-view resolution which is 40 km at nadir, for a swath of 2200 km, allowing global coverage twice a day at 9:30 am/pm local time.

Figure 1 shows the periods for which each of the three successive Metop satellite provided data to generate the MT-CO₂ and MT-CH₄ products..

Figure 1: This diagram illustrates the availability of L2 MT-CO₂ and MT-CH₄ products for the three Metop-A, -B and -C platforms between July 2007 and December 2023

The retrieved CO₂ and CH₄ integrated columns are weighted to the tropical mid-troposphere with peak sensitivity at about 230 hPa (~11 km), half the peak sensitivity at 100 and 500 hPa (~6 and 16 km), and no sensitivity to the surface. These mid-tropospheric column-averaged air mixing ratios (mole fractions) of CO₂ and CH₄ are respectively denoted as MT-CO₂ (in parts per million, ppm) and MT-CH₄ (in parts per billion, ppb). Retrievals are performed over land and sea, by night and day (9:30 am/pm local time) for clear-sky only (no clouds, no aerosols). The CO₂ retrievals are limited to the tropical air masses (between about 30°S:30°N). The CH₄ retrievals should not be used outside the 60°S:60°N latitudinal bands.

The Level 3 data products are in Obs4MIPs format and described in detail in Section 1.1 for MT-CO₂ and in Section 1.2 for MT-CH₄. Obs4MIPs (Observations for Model Intercomparisons Project) https://pcmdi.github.io/obs4MIPs/ (last access: 16-03-2025) is an activity to make observational products more accessible especially for climate model intercomparisons.

The MT-CO₂ and MT-CH₄ Obs4MIPs products are gridded data products in NetCDF format with a spatial resolution of 1^ox1^o (i.e., using an equirectangular (Cartesian) latitude/longitude grid) and daily time resolution (see ATBD MTGHG, 2024). The MT-CO₂ and MT-CH₄ Obs4MIPs products are updated every 6 months.

 Table 1 provides an overview of the L3 MT-CO₂ and MT-CH₄ data Obs4MIPs products  

Table 1: Overview MT-CO₂ and MT-CH₄ Obs4MIPs data products.

The L2 data used as input to generate the MTCO2_OBS4MIPs and MTCH4_OBS4MIPs products are illustrated in Section 1.1 and Section 1.2, respectively.

1.1. Obs4MIPs MT-CO₂ description

The main quantity / data field of this product is the mid-tropospheric column-average air mole fraction of atmospheric carbon dioxide (CO₂), denoted as MT-CO₂.

• L3 MT-CO₂ Obs4MIPs products are computed from L2 version 10.1 MT-CO₂ and MT-CH₄ products from IASI and AMSU-A observations onboard the three Metop platforms (see Figure 1 for MT-CO₂ product availability by platform);
• L2 MT-CO₂ products were produced from IASI L1c and AMSU-A L1b radiance products provided by EUMETSAT through the EUMETCast system;
• L2 MT-CO₂ products are retrieved using the Non Linear Inference Scheme (NLIS) algorithm from CNRS-LMD: version 10.1 for MT-CO₂;
• No bias correction is applied to the L2 MT-CO₂;

For more detailed information on the input data and the processing algorithm see ATBD MTGHG, 2024.

1.2. Obs4MIPs MT-CH₄ description

The main quantity / data field of this product is the mid-tropospheric column-average air mole fraction of atmospheric methane (CH₄), denoted as MT-CH₄.

• L3 MT-CH₄ Obs4MIPs products are computed from L2 version 10.2 MT-CH₄ products from IASI and AMSU-A observations onboard the three Metop platforms (see Figure 1 for MT-CH₄ product availability by platform);
• L2 MT-CH₄ products were produced from IASI L1c and AMSU-A L1b radiance products provided by EUMETSAT through the EUMETCast system;
• L2 MT-CH₄ products are retrieved using the Non Linear Inference Scheme (NLIS) algorithm from CNRS-LMD: version 10.2 for MT-CH₄;
• No bias correction is applied to the MT-CH₄

For more detailed information on the input data and the processing algorithms, see ATBD MTGHG, 2024.

1.3. Overview of Product Target Requirements

Target requirements are described in the Target Requirements and Gap Analysis document (TR GAD GHG, 2024).

The achieved data quality of the data products including comparison with the Target Requirements (TR) is described in Section 5 of document Product Quality Assessment Report (MTGHG PQAR, 2024) and reported in Table 2.

Table 2: Compliance with User Requirements. MT-CO₂ and MT-CH₄ Obs4MIPs random (“precision”), systematic error and stability requirements (from TRD GAD GHG, 2024). Abbreviations: G=Goal (green), B=Breakthrough (yellow), T=Threshold requirement (red). ^§) Required systematic error after an empirical bias correction, that does not use the verification data. ^#) Required systematic error and stability after bias correction, where bias correction is not limited to the application of a constant offset / scaling factor

1.4.  Summary data quality for Level 3 MT-CO₂ and MT-CH₄ products

The validation of Level 3 product MTCO2_OBS4MIPS can be summarized as follows:

• The overall monthly mean uncertainty is 1.25 ppm and the mean bias is 0.6 ppm. This value is based on the comparison between partial column (MT-CO₂) and point measurement (Aircraft measurements).
• Relative systematic error, i.e., the spatio-temporal error, is 1.42 ppm (1-sigma) between 30°S:30°N and 0.52 ppm (1-sigma) between 20°S and 20°N. These values are based on the comparison between partial column (MT-CO₂) and point measurement (Aircraft measurements). The computed linear drift of 0.005±0.04 ppm/year (1-sigma) is small and not significant.
• Overall, this product has therefore reasonable accuracy and high stability.

The validation of Level 3 product MTCH4_OBS4MIPS can be summarized as follows:

• The overall monthly mean uncertainty is 17.3 ppb and the mean bias is -1.81 ppb. Relative systematic error, i.e., the spatio-temporal error, is 3.80 ppb (1-sigma).
• Overall, this product has therefore reasonable accuracy.
• The linear drift could not be computed (for more details, see MTGHG PQAR, 2024).

1.5. Example visualization of key variables

Figure 2 shows the monthly averaged maps of L3 MT-CO₂ (a) and L3 MT-CH₄ (b) for August 2020. L3 MT-CO₂ products are limited to tropical airmasses, typically between around 30°S:30°N (Figure 2(a)). 

For L3 MT-CH₄ products (Figure 2(b)), the high values of MT-CH₄ are seen over the tropics and northern mid-latitudes. MT-CH₄ values decrease in southern mid-latitudes. There is a high concentration of MT-CH₄ over Asia during the monsoon; the CH₄ emitted by rice paddies, combined with strong convections, rapidly transports CH₄  into the mid-troposphere where it can be observed by IASI infrared sounders.

Figure 3 shows the monthly evolution displayed as a function of latitude of L3 MT-CO₂ and MT-CH₄ retrieved from IASI. Figure 4 shows the seasonal maps of L3 MT-CO₂ Obs4MIPs. The same information but on MT-CH₄ may be found in Figure 5. Figure 4 and 5 clearly show the specific seasonal pattern of methane in the free troposphere. Also, the large and regular increase of both CH₄ and CO₂ in the mid-troposphere is well seen over the 16 years. The figures highlight that the increase impacts the entire globe and seasons homogeneously. 

Figure 2: Examples of maps of MTCO2_OBS4MIPS (a) and MTCH4_OBS4MIPS (b) for August 2020

Figure 3: Monthly evolution displayed as a function of latitude of CO₂ (a) and CH₄ (b) mid-tropospheric column averaged mole fraction from L3 MT-CO₂/MT-CH₄ Obs4MIPs.

Figure 4: Seasonal maps of MT-CO₂ Obs4MIPs (v10.1) for January-February-March (JFM, 1^st column), April-May-June (AMJ, 2^nd column), July-August-September (JAS, 3^rd column) and October-November-December (OND, 4^th column).

Figure 5: Seasonal maps of L3 MT-CH₄ Obs4MIPs (v10.2) for January-February-March (JFM, 1^st column), April-May-June (AMJ, 2^nd column), July-August-September (JAS, 3^rd column) and October-November-December (OND, 4^th column).

1.6. Data usage information

The daily NetCDF (version netcdf=4.8.1, hdf5=1.10.7, Conventions = CF-1.7 ODS-2.1) files contain the level 3 mid-tropospheric column averaged mole fractions of CH₄ (ppb) and CO₂ (ppm), which are stored as the mtch4 and mtco2 variables. Averaging kernels are provided on pressure layers (pressure-weight), as opposed to levels. Averaging kernels provide the CO₂/CH₄ vertical sensitivities of MT-CO₂/CH₄ Obs4MIPs retrievals. They are used for the validation and the comparisons of the MT-CO₂/CH₄ Obs4MIPs with independent source of validation by applying these vertical sensitivities to CO₂/CH₄ profiles measured at the ground to obtain mid-tropospheric columns of CO₂/CH₄ like Obs4MIPs.

1.6.1. Data format and file naming 

The MTCO2_OBS4MIPS and MTCH4_OBS4MIPS products are presented in the form of daily NetCDF files with the name convention:

• mtco2_day_C3S-MTCO2-v10.1_BE_gn_YYYYMMDD.nc for MTCO2_OBS4MIPS;
• mtch4_day_C3S-MTCH4-v10.2_BE_gn_YYYYMMDD.nc for MTCH4_OBS4MIPS;
• YYYYMMDD defines the date of the product.

 For example, the product named mtco2_day_C3S-MTCO2-v10.1_BE_gn_20080403.nc refers to the L3 MTCO2_OBS4MIPS product version 10.1 for the 3^rd April 2008.

1.6.2. Quality flags and data masks 

The products contain no quality flags. Missing data is marked for each variable as described in Section 1.5.3.

1.6.3. File contents

Table 3 and Table 4 list the main characteristics of the MTCO2_OBS4MIPS and MTCH4_OBS4MIPS products, respectively. See ATBD MTGHG, 2024, for an overview of how these products have been generated and for additional details.

Table 3: Main characteristics of the MTCO2_OBS4MIPS v10.1 product.

Table 4: Main characteristics of the MTCH4_OBS4MIPS v10.2 product.

Table 5 lists the variables available in the NetCDF MTCO2_OBS4MIPS and MTCH4_OBS4MIPS products files.

Table 5: Variables available in the NetCDF MTCO2_OBS4MIPS and MTCH4_OBS4MIPS products files.  x, y, z, t, represent the number of grid points in longitude, latitude, pressure, and temporal dimension respectively.

Description of each parameter:

time: Time center in days since 1990-01-01. 

time_bnds: Time boundaries. Start and end time of each month in days since 1-Jan-1990 

lat: Latitude center in degrees north (from -90.0º to +90.0º). 

lat_bnds: Latitude boundaries (upper and lower boundaries of 5º latitude bands) in degrees north. 

lon: Longitude center in degrees east (from -180.0º to +180.0º).

lon_bnds: Longitude boundaries (upper and lower boundaries of 5º longitude bands) in degrees east.

pre: Pressure level center, dimensionsless as normalized to surface pressure. 

pre_bnds: Pressure layer boundaries, dimensionsless as normalized to surface pressure. 

mtghg = {mtco2 or mtch4}: Main parameter: Retrieved mid-tropopsheric column of atmospheric CO₂ or CH₄, respectively. Typical values are << 1.0 (typically close to 0.0004 for MT CO₂ and close to 0.0000018 for MT CH₄) and 1.0E20 = no data. 

mtghg_nobs = {mtco2_nobs or mtch4_nobs}: Number of individual MT-GHG L2 observations per grid box used to compute the reported Level 3 XGHG daily average value (0 = no data).

mtghg_std = {mtco2_std or mtch4_std}:  Standard deviation of the L2 observations within each grid box. 

column_averaging_kernel: MT-GHG averaging kernel (dimensionless); a vertical profile (1.0E20 = no data). The normalized column-averaging kernel represents the sensitivity of the retrieved mid-tropospheric GHG to the true mole fraction depending on pressure (height). All values represent layer averages within the corresponding pressure levels. 

1.6.4. Examples of known climate applications and best practices

The Level 3 MT-CO₂ and MT-CH₄ Obs4MIPs products have been primarily generated for comparison with climate models but have also been used for other applications such as computations of annual mean atmospheric growth rates.

Level 3 data are new and thus never used before. Level 2 MT-CO₂ and MT-CH₄ products have been used for flux inversions (Cressot et al., 2014), assimilation within C-IFS (Massart et al., 2014, Agustí-Panareda et al. 2023) or study of methane in the Arctic.

1.6.5. Known Issues and Limitations

For MT-CO₂ Obs4MIPs products:

• The retrievals are limited to Tropical air masses (between about 30°N and 30°S);
• The uncertainties are not provided;

For MT-CH₄ Obs4MIPs products:

• The retrievals are limited to Tropical and Mid-latitude air masses;
• The MT-CH₄ Obs4MIPs retrievals should not be used outside the 60°S:60°N latitudinal bands.
• The uncertainties are not provided;
• The stability of MT-CH₄ Obs4MIPs product could not be calculated from the validation using aircraft measurements.

2. Data access information

The data products and corresponding documentation are / will be made available via the Copernicus Climate Data Store (CDS): https://cds.climate.copernicus.eu/#!/home

Data can be downloaded from the website and used under the License to Use Copernicus Products (included on the download pages).

Direct link to CO₂ products: https://cds.climate.copernicus.eu/datasets/satellite-carbon-dioxide?tab=overview, DOI: 10.24381/cds.f74805c8

Direct link to CH₄ products: https://cds.climate.copernicus.eu/datasets/satellite-methane?tab=overview, DOI: 10.24381/cds.b25419f8

To download the data products, users have to select:

• Processing level: Level 3
• Variable: Column-average air mole fraction
• Sensor and algorithm: IASI and Obs4MIPs

Version: 10.1 (MT-CO₂) and 10.2 (MT-CH₄)

User support is provided by ECMWF via the ECMWF Support Portal. Questions may also addressed through the user forum. 

For specific technical aspects concerning the MT-CO₂ and MT-CH₄ products, users can contact nicolas.meilhac@lmd.ipsl.fr (9am-5pm weekdays). 

Acknowledgments

We acknowledge previous funding by the European Space Agency (ESA) via Climate Change Initiative (CCI) project GHG-CCI. This funding enhanced the quality of the retrieval algorithms and related documentation. This resulted in more mature data products as needed for an operational project such as the Copernicus Climate Change Service (C3S). We also acknowledge the ESPRI computing center.
The development of retrieval algorithms based on IASI observations would have not been possible without the strong support of CNES.
We thank the AirCore-Fr team for providing AirCore data and CONTRAIL team for providing aircraft measurements of greenhouse gases.

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PQAR MTGHG, 2024: Crevoisier, C., Meilhac, N., C3S Greenhouse Gas (GHG: MTCO2 v10.1 & MTCH4 v10.2): Product Quality Assessment Report (PQAR)

TRD GAD GHG, 2024: Buchwitz, M., et al., Target Requirement and Gap Analysis Document, Copernicus Climate Change Service (C3S) project on satellite-derived Essential Climate Variable (ECV) Greenhouse Gases (CO₂ and CH₄) data products (project C3S2_312a_Lot2), Version 5.6, 22-Mar-2024, pp. 87, 2024. Latest version: http://wdc.dlr.de/C3S_312b_Lot2/Documentation/GHG/C3S2_312a_Lot2_TRD-GAD_GHG_latest.pdf

Werschek, 2015: Werschek, M., EUMETSAT Satellite Application Facility on Climate Monitoring, C3S Climate Data Store workshop, Reading, UK, 3-6 March 2015. https://www.ecmwf.int/sites/default/files/elibrary/2015/13546-existing-solutions-eumetsat-satellite-application-facility-climate-monitoring.pdf

Wofsy et al. 2012: Wofsy, S. C., Daube, B. C., Jimenez, R., et al.: HIPPO Merged 10-second Meteorology, Atmospheric Chemistry, Aerosol Data (R 20121129), Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA, https://doi.org/10.3334/CDIAC/HIPPO_010 (Release 29 November 2012), 2012.

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